1. Field of the Invention
This invention relates in general to a method and apparatus for manufacturing of crystalline materials and more particularly to the manufacturing of silicon seed rods for Siemens and similar processes, which are the main methods for manufacturing high-purity polycrystalline silicon and silicon for solar cells.
2. Description of Related Art
In the Siemens process of manufacturing polycrystalline silicon for the semiconductor industry and solar cells, polycrystalline silicon is manufactured by the deposition or growth of silicon on the surface of a solid silicon rod, referred to as a silicon seed rod or core.
As described in U.S. Patent Application Publication No. 2011/0220283 A1 on a disclosure of Akimichi Nagaura et al., the major trends in conventional methods for manufacturing high-purity polycrystalline silicon (SEG-Si) are the Siemens method and the monosilane method, in which a starting-material silane gas is supplied to the interior of a high-temperature reaction furnace, and polycrystalline silicon is manufactured by the deposition or growth of silicon on the surface of a solid silicon rod (silicon seed rod or core). According to this producing method, a plurality of silicon seed rods are provided in a reactor and heated, feed gas is supplied to the reactor and brought into contact with the heated silicon seed rods, and polycrystalline silicon is deposited on the surface of the silicon seed rods by heat decomposition and hydrogen reduction of the feed gas to grow columnar-shaped silicon rods. The silicon seed rods are fixed upright on electrodes that are mounted on a bottom plate portion of the reactor. Every two silicon seed rods are paired and the upper end portions thereof are connected to each other by a short connection rod, so that they form a U-shape.
The silicon seed rods are manufactured by hewing out from a silicon rod. Therefore, since the silicon seed rods are getting shorter after every manufacturing and hewing out cycle, it is necessary to manufacture silicon seed rods from long silicon seed rods periodically. The long silicon seed rod can be produced, for example, by joining short silicon seed rods by welding. See Japanese Unexamined Patent Application, First Publication # S63-242339, and U.S. Patent Application Ser. No. 2011/0220283 A1.
Since the diameter of the silicon rod on which the silicon polycrystals are deposited following the pyrolysis is extremely slender at around 5 mm, the surface area available for deposition in the initial stage of the reaction is so small that this method suffers from the drawback of a slow deposition rate. Furthermore, the resistivity of the silicon rod is high, i.e., 1 kiloohm-cm or higher so that is difficult to pass current through the rod at room temperature. Accordingly, when the reaction is initiated, it is necessary to heat the seed rod from outside by means of a preparatory heating device up to a temperature that enables heating by passage of electrical current. Not only is a high-voltage power supply device required for this heating, but a large amount of electric energy is consumed. This is a factor that increases the cost.
A method using a slender core rod comprising a metal, such as Mo, W, Ta, Nb or the like, with a high recrystallization temperature instead of the Si seed rod used in the Siemens method is also known (U.S. Pat. No. 7,732,012 B2). However, in case of SEG-Si obtained by using a slender core rod comprising metals, such as Mo, W, Ta, Nb or the like, the core part must be removed by some method following completion of the reaction. Moreover, these metals cause diffusion into the silicon that is deposited and grown.